1. Field of the Invention
The present invention relates to a capacitance type moisture sensor having the capability of accurately detecting amounts of water, and a method of producing the same sensor.
2. Disclosure of the Prior Art
Capacitance type moisture sensor is known as a kind of conventional moisture sensor, and detects amounts of water from a change of capacitance between a pair of electrodes in accordance with the nature that water is a polarization (dielectric) material.
For example, this type of moisture sensor is built in a garbage disposing apparatus, and used to detect amounts of water in the garbage. That is, as shown in FIGS. 21A and 21B, the moisture sensor comprises an electrically-insulating panel 2P, which is attached to an opening formed in a garbage vessel 100P of the garbage disposing apparatus, a pair of electrodes 30P, 35P arranged in parallel on the electrically-insulating panel, a capacitance detecting circuit 40P for detecting a capacitance value (Cx) between the electrodes, and an output circuit 42P for providing an electrical signal corresponding to the amounts of water according to outputs from the capacitance detecting circuit.
In this moisture sensor, the amounts of water (M) is represented by the equation (1):
M=V/(S*d)xe2x80x83xe2x80x83(1)
wherein xe2x80x9cVxe2x80x9d is a volume of water, xe2x80x9cSxe2x80x9d is an area of the electrode, xe2x80x9cdxe2x80x9d is a distance between the electrodes. Therefore, xe2x80x9cS*dxe2x80x9d gives a volume of a moisture detection region. A relationship between the amounts of water (M) and the capacitance (Cx) is shown in FIG. 22. This relationship is also expressed by the following equation (2):
Cx=[∈1xe2x80x2(water)*M+∈2xe2x80x2(others)*(1xe2x88x92M)]xc3x97∈xe2x80x20*S/dxe2x80x83xe2x80x83(2)
wherein (∈1xe2x80x2) is the dielectric constant of water (=80) and (∈2xe2x80x2) is a relative dielectric constant of a material other than water. For example, when the material other than water is woods, the relative dielectric constant (∈2xe2x80x2) is 2. When the material other than water is the air, the relative dielectric constant (∈2xe2x80x2) is 1. Thus, the capacitance (Cx) can be determined by the amounts of water in the detection region. In addition, the capacitance value (Cx) depends on the electrode size.
However, there is still plenty of room for improvement in the conventional moisture sensor from the following viewpoints. That is, since these electrodes project toward the interior of the garbage vessel, a space between the electrodes is easily filled with the garbage. In this case, the capacitance detecting circuit may detect only the amounts of water of the garbage caught between the electrodes. Therefore, to accurately detect a change of the amounts of water of the garbage in the garbage vessel, a cleaning mechanism for frequently removing the garbage caught between the electrodes is needed. This leads to an increase in production cost.
In addition, since the garbage is usually stirred in the garbage vessel to facilitate decomposition of the garbage, there is a problem that a breakage of the moisture sensor is accidentally caused by a collision of the garbage with the electrodes. Moreover, the electrically-insulating panel supporting the electrodes needs a mechanical strength sufficient for withstanding a load for stirring. This also leads to a further increase in production cost.
Therefore, a primary object of the present invention is to provide a capacitance type moisture sensor, which can accurately detect amounts of water of an object in a real-time manner without using the above-described cleaning mechanism for electrodes.
That is, this moisture sensor comprises a sensor housing having a electrically-insulating wall, a pair of electrodes disposed on the sensor housing, and a circuit unit. An outer surface of the electrically-insulating wall faces a space where amounts of water should be detected. On an inner surface of the electrically-insulating wall, at least one of the electrodes is formed. An electric field developed between the electrodes is defined as a moisture detecting region. The circuit unit includes a capacitance detecting circuit for detecting a capacitance value between the electrodes, which changes in response to the amounts of water in the moisture detecting region, and an output circuit for providing an electrical signal corresponding to the amounts of water according to the capacitance value detected by the capacitance detecting circuit.
With respect to the moisture sensor of the present invention, since the electric field developed between the electrodes provides the moisture detecting region outside the sensor housing through the electrically-insulating wall, it is not necessary to allow the electrodes to project toward the space where the amounts of water should be detected. In particular, when the moisture sensor is used in a garbage disposing apparatus, it is possible to eliminate the problem that the breakage of the moisture sensor is caused by the collision of the garbage with the electrodes, and accurately detect the amounts of water in the garbage in the real-time manner, while preventing the increase in production cost.
In the above moisture sensor, it is preferred that at least one of the electrodes is a metal film deposited on the inner surface of the electrically-insulating wall. In this case, even when the electrically-insulating wall has a curved surface, good adhesion strength between the electrode(s) and the electrically-insulating wall can be obtained. In addition, it is possible to easily form the electrodes on the sensor housing having a complex shape. Moreover, even when a plurality of ribs described later are formed on the inner surface of the electrically-insulating wall, it is possible to easily form a desired electrode pattern. Alternatively, the electrodes may be formed by means of an insert molding, or metal bonding.
With respect to the above moisture sensor, it is preferred that the sensor housing has a concave formed in an inner surface of a bottom wall thereof and a convex formed on an outer surface of the bottom wall at a position opposed to the concave, and a bottom surface of the concave is the inner surface of the insulating wall and a top surface of the convex is the outer surface of the insulating wall. In this case, for example, when the moisture sensor of the present invention is attached to a garbage vessel of the garbage disposing apparatus, only the top surface of the convex is exposed to the interior of the garbage vessel. Therefore, it is possible to minimize an area of the moisture sensor exposed to the garbage put in the garbage vessel. In addition, since the moisture sensor is attached to the garbage vessel such that the top surface of the convex is flush with the inner surface of the garbage vessel, it is possible to avoid a situation that a movement of the garbage in the garbage vessel by stirring is obstructed, or a situation that an excessive force is loaded to the moisture sensor by the stirring.
With respect to the above moisture sensor, it is preferred that the pair of electrodes are composed of a ring-shaped electrode having an open center and a central electrode, which is disposed in the open center of the ring-shaped electrode such that a circumference of the central electrode is spaced from an inner circumference of the ring-shaped electrode by a constant distance. In particular, it is preferred that the pair of electrodes are composed of a ring-shaped electrode having an annular open center and a central electrode having a circular shape, and the ring-shaped electrode is disposed in a concentric manner with respect to the central electrode such that an outer circumference of the central electrode is spaced from an inner circumference of the ring-shaped electrode by a constant distance. Alternatively, it is preferred that the pair of electrodes are composed of a ring-shaped electrode having a rectangular open center and a central electrode having a rectangular shape, which is disposed in the open center of the ring-shaped electrode such that an outer circumference of the central electrode is spaced from an inner circumference of the ring-shaped electrode by a constant distance. In these cases, since the distance between the electrodes is constant over the entire circumference of the central electrode, it is possible to obtain the same density of electric flux line on the respective electrodes, and stably provide further accurate detection of the amounts of water by reducing the stray capacitance.
With respect to the above moisture sensor, it is preferred that the sensor housing has a plurality of first ribs projecting on the inner surface of the electrically-insulating wall of the concave, which extend in directions of electric field developed between the electrodes. In other words, it is preferred that the sensor housing has a plurality of first ribs projecting on the inner surface of the electrically-insulating wall of the concave, which radially extend from a center of the central electrode. In addition, it is preferred that the sensor housing has at least one second rib projecting on the inner surface of the electrically-insulating wall of the concave such that an extending direction of the second rib is substantially perpendicular to a direction of electric field developed between the electrodes. In other words, it is preferred that the sensor housing has at least one second rib projecting on the inner surface of the electrically-insulating wall of the concave, which extends in a concentric manner with respect to the central electrode. In these cases, it is possible to prevent the non-uniformity of electric-field distribution by forming the first ribs radially extending from the center of the central electrode and the second rib(s) extending in the concentric manner with respect to the central electrode, to thereby accurately detect the amounts of water. In addition, the mechanical strength of the electrically-insulating wall can be improved. Moreover, when the electrodes are connected to a printed circuit board mounting electronic parts of the control unit by soldering a lead wire therebetween, the ribs can work as a barrier for stopping a flow of the solder. These ribs also work as guide means that facilitates the positioning of the electrodes.
With respect to the above moisture sensor, it is preferred that a width of the ring-shaped electrode is substantially equal to the width of the central electrode. In this case, since amounts of electric flux line provided from one of the electrodes are equal to the amounts of electric flux line provided from the other electrode, it is possible to reduce the influence of stray capacitance.
With respect to the above moisture sensor, it is preferred that the circuit unit comprises a signal processing means for detecting an unusual state by comparing a change of the amounts of water corresponding to the electric signal provided from the output circuit with a predetermined value. In this case, it is possible to detect the occurrence of such an unusual state in the space where the amounts of water should be detected, and give an alarm to the user.
With respect to the above moisture sensor, it is preferred that an exposed surface of each of the electrodes is coated with an electrically-insulating material. In addition, after the pair of electrodes are formed on the bottom of the concave of the sensor housing, it is preferred that an electrically-insulating material is filled in the concave. It is useful to prevent a situation that a short circuit between the electrodes is caused by condensation.
In addition, it is preferred that the above moisture sensor comprises a shied case composed of a bottom wall having an aperture and side walls projecting upward from the circumference of the bottom wall, which is disposed in the sensor housing such that the concave of the sensor housing is exposed to the interior of the shield case through the aperture, and the shield case is connected to a zero-voltage point of the capacitance detecting circuit. In particular, it is preferred that the above moisture sensor comprises a shield cover disposed in the sensor housing, and a printed circuit board mounting electronic parts of the circuit unit is housed between the shield case and the shield cover. By using the shield cover and shield case, electric wave that comes from the outside is shielded to prevent a faulty operation of the moisture sensor. In addition, since a leakage of electric wave from the moisture sensor is prevented, it is possible to avoid a bad influence to an electric appliance outside.
In addition, it is preferred that the above moisture sensor comprises an electric wave shield layer formed by depositing a metal material on an internal surface of the sensor housing other than the concave, and the shield layer is connected to a zero-voltage point of the capacitance detecting circuit. In particular, it is preferred that electrical connections between the electrodes and a printed circuit board mounting electronic parts of the circuit unit, and between the electric wave shield layer and the printed circuit board are made by use of a metal film deposited. In these cases, when depositing the metal material on the sensor housing to form the electrodes, it is possible to simultaneously form metal films for the electric wave shield layer and the electrical connections. Therefore, the production cost of the moisture sensor can be remarkably reduced.
A preferred method of producing the above moisture sensor comprises the steps of molding a synthetic resin material to obtain the sensor housing; depositing a metal material to form a metal film on the sensor housing; and selectively removing the metal film from the sensor housing to obtain the pair of electrodes. According to this method, it is possible to simultaneously form the electric wave shield layer, the metal films for the electrical connections and the electrodes on the sensor housing.